1. Field of the Invention
The present invention relates to a receiver, and more particularly to a direct conversion receiver used for various radio communication devices such as selective call receivers.
2. Description of the Related Art
A receiving system called to as direct conversion system in which a signal carried by a carrier frequency is directly frequency-converted into a baseband signal has recently been put to practical use in order to provide compact and low power consumption radio communication devices, particularly Frequency-Shift Keying (FSK) signal receivers. FIG. 3 shows an exemplary structure of such an FSK signal receiver using the direct conversion system (refer to, for example, Proceedings for Spring Cession of National Conference by the Electronic Information and Communication Engineers Society (Denshi Joho Tsushin Gakkai), No. 2232, 1987; Kohji CHIBA et al.: "Performance of Direct Conversion FSK Receiver"). In FIG. 3, a reference numeral 1 denotes an antenna; 2 a radio frequency amplifier; 3 and mixers; 5 and 6 low pass filters; 7 and 8 amplitude limiting amplifiers, 9 a detector; 10 a phase shifter; and 11 an oscillator.
In operation of the above-mentioned FSK signal receiver, an FSK signal is received by the antenna 1. After the received signal is amplified by the radio frequency amplifier 2, it is distributed to the mixer 3 and 4. A local oscillation signal having a frequency which is substantially equal to the carrier frequency of the FSK signal is generated by the local oscillator 11 and then fed to the phase shifter 10. The phase of the local oscillation signal is shifted by 90.degree. by the phase shifter 10 to generate two signals which are orthogonal in phase to each other for the mixers 3 and 4. The two orthogonal signals are mixed with the FSK signal by the mixers 3 and 4 to output I and Q signals, respectively, which are FSK demodulation signals with 90.degree. out-of-phase to each other. After the I and Q signals are passed through the filters 5 and 6, respectively, they are amplified by the amplitude limiting amplifiers 7 and 8. The FSK signal is demodulated from the amplified I and Q signals by the detector 9.
In FIG. 4, a reference numeral 15 denotes a conventional orthogonal mixer unit including the mixers 3 and 4 and RF denotes a carrier signal. The mixer unit 15 is comprised of a semiconductor integrated circuit which is encircled by a dotted line. Reference numerals in FIG. 4 which are like to those in FIG. 1 denote like components. The orthogonal mixer unit 15 is adapted to receiver the carrier signal RF and the local oscillation signals, the phases of which have been shifted by the phase shifter 10, and to supply the I and Q signals.
The direct conversion system has a feature that the carrier frequency of the received FSK signal is substantially equal to the frequency of local oscillation signal. Accordingly, there is the possibility that a local oscillation signal radiated from one receiver is received by the other receiver having the same reception frequency located in the vicinity of the first receiver to mask a normal FSK signal. There is also the possibility that in case of pager and the like having a built-in receiving antenna, the local oscillation signal is leaked to the antenna to mask the normal FSK signal since the tuning frequency of the antenna is substantially equal to the frequency of the local oscillation signal. Radiation of the local oscillation signal from the mixer unit, the radio frequency amplifier and the antenna will interfere with the other receivers. If a FSK signal having a high input level is received by the antenna, the high level FSK signal will unwantedly reach at the oscillator through a path from an FSK signal input terminal of the mixer unit to an local oscillation signal input terminal, resulting in no oscillation of the oscillator.
Accordingly, an approach that an oscillator is implemented in an unit by being housed by, for example, a shield case has been adapted for the structure of the receiver in order to prevent the local oscillation signal from being leaked.
If a mixer unit is formed on a semiconductor integrated circuit in the direct conversion receiver having the structure as shown in FIGS. 3 and 4, the mixer unit includes, for example, a signal or double balanced mixer. The above-mentioned receiver usually requires the input level of the order from -10 to 0 dBm to the mixers 3 and 4 from the oscillator 11 to obtain necessary performances. It is necessary to input a high level local oscillation signal to a lead terminal of the semiconductor integrated circuit on which the mixer unit is provided. Therefore, in order to prevent the high level oscillation signal from being leaked, it is necessary to house not only the oscillator but also the lead terminals of the mixer unit in the shield case. In this way, prior art direct conversion receivers have problems that the packaging of the components in the receivers becomes complicated and that high and sophisticated technology to suppress the leakage of the local oscillation signal is needed.